I wouldn't normally bother to grind my own, unless (a) the recipe called for "freshly ground", or (b) I had no pre-ground but I did have seeds. That said, I have just tried grinding coriander seed in a
Kenwood coffee grinder, grinding for periods of 15 seconds and then resting the grinder so as not to overheat it. After 15 seconds, I had coarse sawdust; after two minutes, I could see no further improvement (from 1:45, that is), so stopped at that point. The resulting ground spice was slightly coarser than my pre-ground coriander, but it is possible that commercial spice grinders also sieve as the final stage to remove any remaining husk. I would have no reservations about using this after 2 minutes of grinding in lieu of pre-ground coriander, and the smell released by the essential oils during the grinding process really made the freshly ground version seem more appetising.
** Phil.